The Monitor has both audible and visual alarms. 2 built-in relays can trigger the alarms at different user-defined CO2 levels, as well as control a ventilation fan or an HVAC control switch to notify maintenance staff of high CO2 levels. This meets the OSHA PEL (Personal Exposure Limit) Standard of 5,000ppm TWA (time weighted average) in a typical 8-hour workday.